1. Field of the Invention
This invention is directed to a calcium carbonate composition containing a minor amount of kaolin clay and to the use of the composition in organic media, particularly rubber matrices.
2. Description of the Prior Art
Calcium carbonate, particularly when in a finely divided state, has many desirable properties which make it valuable as a reinforcing pigment in various organic compositions, as, e.g., plastics, paints, rubbers, etc. A fine particle size calcium carbonate is particularly useful in rubber stocks where it reinforces the rubber and gives rise to high tensile strengths. Because calcium carbonate is pure white in color, it is possible to prepare high tensile strength light colored rubber stocks, which is not possible with many reinforcing pigments such as carbon black.
In spite of the many desirable properties of calcium carbonate, it has certain serious shortcomings which limit its acceptance by the rubber industry. The ultra fine particle size calcium carbonate forms hard agglomerates which are extremely difficult to incorporate and disperse in the rubber matrix. When such a fine particle size calcium carbonate is incorporated into rubber on a two-roll differential speed rubber mill, the calcium carbonate pigment forms a hard cake on the back roll which must be loosened by scraping, and the resulting agglomerated particles disperse poorly in the rubber. The aforementioned difficulties can be partially overcome by master batching techniques, but these require longer processing cycles and limit the acceptance of calcium carbonate in the rubber industry.
It is particularly important in industry that the calcium carbonate should disperse quite readily in water. Aqueous dispersions of calcium carbonate find utility in the form of slurries for paper products, for rubber latex formulations, as well as for other purposes. It has often been found quite difficult to disperse the calcium carbonate in water, and to achieve slurries of an appropriate concentration without unduly increasing the slurry viscosity. It is of course a material advantage to be able to produce a concentrated slurry provided the viscosity is such that the slurry can be flowed and otherwise handled with convenience. Another disadvantage encountered in prior art aqueous slurries of calcium carbonate materials is that the materials frequently settle and pack in the bottom of their containing vessels. The settled particles can pack so hard that it is almost impossible to redisperse them.